bad urls for "Delta Chat" talk from 36c3

[unwiredben]

for talk ID 18a492b2-6219-5b95-a48d-b03944b75739, "Delta Chat: e-mail based messaging, the Rustocalypse and UX driven approach [YBTI/wefixthenet session]", several of the URLs are bad -- they're missing the oio folder in the URLs, like the thumb_url that is https://static.media.ccc.de/media/congress/2019/154-hd.jpg.

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[saerdnaer]

Manually fixed in the database, images path prefixes have to be implemented in the publishing script, c.f. voc/voctopublish#82